FIG. 6 is a block diagram showing an electrical construction of a typical conventional switched-mode power supply 1. The power supply 1 supplies multiple loads 11, 12 as multiple secondary circuits with desired constant output voltages vo1, vo2 respectively.
As shown in the figure, in the power supply 1, the AC voltage (AC power) taken from, for example, a commercial power source 2, is fed to the AC input section. The AC voltage is then passed through a rectifier diode d0 and a smoothing capacitor c0 to transform it to a DC voltage on which the power supply 1 itself will operate (alternatively, the power supply 1 may be arranged to operate on a battery or other DC power source).
Still referring to FIG. 6, in the power supply 1, the smoothing capacitor c0 is provided across a switching device q and the primary winding 3a of a transformer 3 which are connected in series.
The series circuit stores magnetic energy in the primary winding 3a when the switching device q is on. The energy is then drawn from the first secondary winding 3b of the transformer 3 through the rectifier diode d1 and smoothed by the smoothing capacitor c1 when the device q goes off. The actions produce a relatively low power (low consumption) output to the first load l1 at the output voltage vo1 of a relatively low value.
In the power supply 1, the output voltage sensor circuit 4 detects the output voltage (DC output voltage) vo1 and provides the result as a feedback to the primary-side switching control circuit 5 through a photocoupler and other devices (not shown in the figure) disposed for insulating purposes. The switching control circuit 5 controls the switching action according to the magnitude of the output voltage vo1 to regulate the output voltage (supply voltage) vo1.
Still referring to FIG. 6, in the power supply 1, DC power is drawn also from the second secondary winding 3c of the transformer 3 through a rectifier diode d2 and smoothed by a smoothing capacitor c2 when the switching device q is off. The actions produce a relatively high power (low consumption) output to the second load l2 at the output voltage vo2 of a relatively high value.
The power supply 1 is adapted to provide the relatively low output voltage vo1 as a feedback to the switching control circuit 5, because the first load l1 to which is supplied a lower voltage is a kind of load that requires the output voltage vo1 to be highly precise and that needs to be fed with the output voltage vo1 even when the electronics are standing by (e.g., microcomputer), whereas the second load l2 to which is supplied a higher voltage is a kind of load that can operate on the less precise output voltage vo2 and that does not need to be fed with the output voltage vo2 when the electronics are standing by (e.g., a motor). Exemplary electronic devices with such two power outputs are printers, copying machines, facsimiles, and other like printing machines.
Still referring to FIG. 6, in the primary side of the power supply 1, there is provided an output value sensor circuit 6 detecting the current flow through the switching device q. The output value sensor circuit 6 senses a voltage value which is in proportion to the total power output (total power) to the secondary circuits (voltage drops, etc. by a resistor (not shown in the figure) and other components connected in series with the device q). The result is given to the switching control circuit 5. This enables the power supply 1 to provide overcurrent protection to the secondary circuits.
Thus, the power supply 1 is adapted to implement overcurrent protection, depending on a value in proportion with the total power output to the secondary circuits.
Suppose, for example, that the first load l1, to which is supplied a lower voltage, requires an electric power at 3.3 V, 5 A, and also that the second load l2, to which is supplied a higher voltage, requires an electric power at 24 V, 2 A. The total power output to the secondary circuits is 3.3×5+24×2=64.5 W. Note that we ignore various power dissipations including the forward voltage drops by the diodes d1, d2 for simple description.
Suppose further that settings are made so that overcurrent protection is activated when the total power output to the secondary circuits reaches 65 W. In this situation, overcurrent protection starts when the current flow through the first load l1 reaches 65/3.3=19.7 A if the high power consuming second load l2 is small (for example, there is no second load l2 or its resistance is extremely high).
In this manner, in the power supply 1, if the second load l2, to which is supplied a higher voltage, is provided with a low power supply, the current flow through the first load l1, to which is supplied a lower voltage, becomes as high as 19.7 A, far exceeding its rating (5 A), before overcurrent protection starts. To avoid such overcurrent, changes should be made to the design: for example, the DC resistance may be lowered, the current rating of the rectifier diode d1 constituting one of the secondary circuits may be increased, or the cross-section of the second secondary winding 3b of the transformer 3 may be increased. Any of the changes can be made only at an additional cost.
The problems can be addressed also by an individual current sensor circuit connected to each of the secondaries' outputs. However, the arrangement inevitably requires that the outputs be terminated or controlled independently from each other based on the two detected current values or together by feeding both the values to the primary. The circuit therefore becomes complex and induces additional cost as in the previous case.